In packet-based information transfer networks, such as the Internet for example, IP local area networks or the like, several modes of transfer of the information are found. These modes may be classed into three categories as a function of the number of senders and of receivers engaged in this transmission. Firstly, there is point-to-point transmission (also known as “unicast”) which allows a sender to dispatch an information packet to a single receiver identified by its address on the network. This is the mode of sending used by the most popular protocols of the Internet network such as the HTTP web page transfer protocol (“Hyper Text Transfer Protocol”) or the FTP file transfer protocol. Another mode of sending consists in a sender transmitting a packet in broadcast mode. In this mode, the packet sent by the sender is sent to all the nodes of the network. This mode is not generally available on the Internet but is found on local area networks. The third mode consists in a sender or group of senders transmitting a packet to a group of receivers, in a multipoint mode of sending (otherwise known as “multicast”). In this mode the packets are sent to an address termed the multipoint transmission address and will be forwarded to all the destinations belonging to the transmission group. A client who joins a transmission group will be said to subscribe to the group and a client who leaves the group will be said to desubscribe from the group.
The multipoint transmission mode is used in practice to save intermediate bandwidth in the network when a source sends data to a group of destinations. Specifically, in this case, the use of a point-to-point mode of sending implies that the data are dispatched as many times as there are destinations. This mode brings about the duplication of the packets on the parts of the network that are common to the paths between the source and the various destinations. Conversely, multipoint transmission makes it possible to dispatch the data just once, this data being duplicated on the routers of the network, as a function of the paths leading to the destinations belonging to the transmission group. FIG. 1a illustrates the transmission of a data packet “P” sent by a node “S”, the information source, to nodes “A”, “B” and “C”. It is seen that packet “P” is duplicated three times between the node “S” and the router “R1”, twice between the routers “R1” and “R2” in the case of point-to-point transfer and is not duplicated in the case of multipoint transfer illustrated by FIG. 1b. In this case, a single packet “P” is sent by the source “S”, the router “R1” knows that a packet must be resent on two branches out of three bound for node “A” and for the router “R2” which itself sends it to the clients “B” and “C” who are members of the group, since the clients A and B, C have previously informed the routers R1 and R2 that they wish to receive the packets of the group P. The packet is not dispatched towards the nodes “D” and “E” which are not members of the transmission group.
A local area network generally comprises a gateway linking the local area network proper and the exterior network, generally the Internet. To this gateway are connected, according to several possible technologies such as Ethernet, IEEE 1394 or technologies for wireless connection by radio, various local devices. These devices can access the exterior network via the gateway operating as a router between the local area network and the exterior network. When a local device, the client, wishes to join a multipoint data transmission group, he subscribes to the multipoint transmission address, for example according to the IGMP Internet Group Management Protocol which protocol is known by the reference “RFC 3376” at the IETF (“Internet Engineering Task Force”). Following this subscription, the node is recognized as a member of the transmission group and the packets corresponding to this stream, and transmitted in multipoint mode are routed from the Internet, via the gateway, to this client.
The streams transmitted in multipoint mode may often be divided into several parts. This is, for example the case when using incremental coders (also known as “scalable codecs”). These coders, a detailed description of which may be found (for example in ISO/IEC 14496-2:1999/FDAM 4, Coding of audio-visual objects—Part 2: Visual, Streaming video profile, Fine granularity scalability or else ISO/IEC JTC1/SC29/WG11, MPEG04/N6372 “Scalable Video Model V 1.0”) are envisaged inter alia by the standard MPEG 4, ref. They have the particular feature of coding a digital service as several streams, a base stream and complementary streams. The base stream on its own makes it possible to reconstruct the service on the client in a degraded manner. For example it will be possible to reconstruct a low-resolution video of the service. While the decoding, of the base stream and of the complementary streams will allow the restitution of the service in its full quality. Non-incremental coders can also generate streams divisible into a base stream and complementary streams. It is for example possible to group the images as a function of their MPEG type (I, P or B) and to undertake temporal dimensioning.
It is found that on the local area network, according to the technology used, the multipoint transmission is not always performed safely. For example. were the local area network to be a wireless network operating according to a protocol of the 802.11 family in version a, b or g, the packet sent is tested to see whether it is intact and those packets that are not intact are discarded but not re-sent. They are lost.